1. Field of the Invention
The present invention relates to an improved solar cell. More particularly, the present invention relates to a solar cell substrate which makes it possible to scatter incident light and to effectively utilize light absorbed by the active layer of a solar cell, thereby stably improving the power output of the solar cell, and an improved solar cell provided with said substrate.
2. Related Background Art
U.S. Pat. No. 4,126,150 discloses a method of improving the power output of a photovoltaic device provided with a light reflective substrate by making the light reflective surface of said substrate an uneven surface provided with irregularities to increase particularly the optical path length of long wavelength light of absorption coefficient (this is termed a "light confining effect"; the term "irregularity" means an intentionally formed surface shape of a substrate providing certain effects, excluding unintentional shapes such as flaws, etc. at the surface of a substrate). Disclosure similar to that in said U.S. patent is found in Unexamined Japanese Patent publication No. 56(1981)-152276, which also includes a description about roughness of the irregularity. Further, Unexamined Japanese Patent Publication No. 59(1984)-104185 (equivalent to U.S. Pat. No. 4,497,974) discloses optical effects of a substrate having an uneven surface.
Further, in Journal of Applied Physics. Vol. 62, No. 7, pp. 3016-3024, mathematical observations on the optical reflection characteristics of the amorphous silicon solar cells in which a textured silver layer is used are described.
As for methods of forming irregularities at the surface of a substrate, a wet etching method by Unexamined Japanese Patent Publication No. 54(1979)-153588 (equivalent to U.S. Pat. No. 4,252,865); a sandblasting method, a facet forming method and a coevaporation method by Unexamined Japanese Patent Publication No. 58(1983)-159383 (equivalent to U.S. Pat. No. 4,419,533); a method of roughening the surface of an aluminum substrate with a D.C. electrolytic etching method or a chemical etching method by Unexamined Japanese Patent Publication No. 59(1984)-14682; a sputter-etching method and a sandblasting method by Unexamined Japanese Patent Publication No. 59(1984)-82778; and a lithography method, a transparent conductor deposition method by means of thermal decomposition spraying, and an ion beam simultaneous deposition method and an etching method by Unexamined Japanese Patent Publication No. 59(1984)-104185 (equivalent to U.S. Pat. No 4,497,974) have been proposed.
Other than the above, with respect to use of a material capable of easily forming irregularities at the surface thereof, an organic insulating layer and a metal reflective layer disposed on said insulating layer by Unexamined Japanese Patent Publication No. 58(1983)-180069; and a ceramic substrate by Unexamined Japanese Patent Publication No. 59(1984)-213174 have been proposed.
In order to prevent a reduction in the yield due to pin holes and the like in the case of forming a Schottky junction or a PIN junction on a reflective substrate, SERI Report San-1286-8 (Carlson et al., Oct. 1978, EY-76-C-03-1286) discloses an advantage of disposing a cermet layer on the reflective substrate.
In addition, in order to prevent occurrence of short circuits due to a flaw or projection on a reflective substrate, Unexamined Japanese Patent Publications Nos. 56(1981)-69875 and 58(1983)-35988 propose intervention of a transparent conductive film.
Further, the above mentioned Unexamined Japanese Patent Publication No. 58(1983)-159383 proposes disposition of a transparent conductive film on the uneven reflective surface of a reflective conductive layer. The transparent conductive film prevents diffusion of the constituent materials of the uneven-shaped reflective conductive layer into the semiconductor layer, which results in deterioration of the characteristics of the semiconductor layer.
Further, the above mentioned Unexamined Japanese Patent Publication No. 59(1984)-104185 (equivalent to U.S. Pat. No. 4,497,974) discloses a technique of improving the light collecting efficiency of a reflective type solar cell in the region of long wavelengths by increasing the optical path by disposing a pair of transparent conductive films, one of which has a roughened surface.
In addition, Unexamined Japanese Patent Publication No. 60(1985)-84888 (equivalent to U.S. Pat. Nos. 4,532,372 and 4,598,306) proposes disposit layer in order to prevent occurrence of short circuits due to pin holes or projections at the upper and lower electrodes.
As described above, there have been a number of proposals in order to improve photovoltaic characteristics of a solar cell. However, various problems still need to be solved in order to provide a desirable solar cell.
For instance, in the case where the light to be incident a solar cell is of substantially short wavelength, such light is effectively absorbed by its optically active layer (that is, the layer which serves to absorb light to generate photocarriers and to transport said photocarriers by way of an internal electric field or by diffusion). There is still not a practical effective solution to efficiently transport the generated photocarriers to the electrode.
Conversely, for the case where the incident light is of long wavelength and is not sufficiently absorbed by the optically active layer, there is still not a practical effective solution to recycle the light which has passed therethrough.
In this regard, there has been a proposal to make the thickness of the optically active layer as thin as possible in the case of a solar cell composed of an amorphous silicon incapable of providing a diffusion length. This proposal is worth discussing in view of improving the photoelectric conversion efficiency by effectively utilizing the long wavelength light.
However, for any of the known amorphous silicon solar cells even when provided with optimized reflective substrates, there are still a number of unsolved problems.
That is, in the case of an amorphous silicon solar cell, for the light of the wavelength to be partly absorbed by the optically active layer, while the remainder passes therethrough and is reflected by the substrate, it is desired that it be reflected at a scattering angle as small as the absorption coefficient is or as large as the wavelength is in order to increase the quantity of light absorbed by the optically active layer.
In the prior art, where it is intended to output a large electric current by increasing the light confining effect, it is common to enlarge the extent of an irregularity at the surface of the substrate. However, enlargement of the irregularity to an excessive extent will cause a short circuit of the solar cell to be produced, resulting in reduction in the yield or poor power output characteristics. problems, it is necessary to use as the reflective substrate a member which satisfies at least the following three items:
(1) reflects light reaching the substrate without being absorbed by the optically active layer at an effective angle,
(2) serves to prevent occurrence of short circuits without causing a reduction in the open circuit voltage and the fill factor and also without causing a reduction in the yield due to occurrence of short circuits during the production process thereof, and
(3) can be easily prepared at a reduced cost.
The present invention has been accomplished based on the above findings.
An object of the present invention is, therefore, to provide an improved reflective substrate provided with specific irregularities having a large difference of circuits when used as the reflective substrate of a solar cell.
Another object of the present invention is to provide an improved reflective solar cell substrate which reflects light reaching the substrate without being absorbed by the optically active layer at an effective angle to thereby exhibit improved photovoltaic characteristics.
A further object of the present invention is to provide an improved solar cell provided with said reflective substrate which exhibits improved photovoltaic characteristics. concave portions. Thus, this solar cell is not satisfactory in view of preventing occurrence of short circuits throughout the solar cell.
In order to eliminate the above problem relating to occurrence of short circuits due to the irregularities at the surface of the substrate, it is effective to provide a transparent electrode of such a thickness that makes it possible to prevent occurrence of short circuits in the solar cell. As a result of studies by the present inventors, it has been concluded that this idea is effective for the purpose of preventing the occurrence of short circuit but causes other problems. That it, in solar cells having a transparent electrode with such thickness, the electrical resistance of the transparent electrode in the thickness direction increases the series resistance of the solar cell, thereby negatively affecting the characteristics of the solar cell, particularly the open circuit voltage and fill factor. Because of this, the resulting solar cell is poor in photoelectric conversion efficiency.